In recent years, attention has focused on preferred degradable polymers which can be converted to desired substrates or articles. Much of this attention is focused on polymers which include, as monomeric units therein, the result of lactic acid or lactide polymerization. Attention is directed, for example, to U.S. Pat. No. 5,142,023 to Gruber et al.; U.S. Pat. No. 5,338,822 to Gruber et al.; U.S. Pat. No. 5,475,080 to Gruber et al.; U.S. Pat. No. 5,359,026 to Gruber; and U.S. Pat. No. 5,594,095 to Gruber et al. the complete disclosures of which are incorporated herein by reference. It is noted that U.S. Pat. Nos. 5,142,023; 5,338,822; 5,475,080; 5,359,026; and 5,594,095 are owned by Cargill Incorporated, of Minneapolis, Minn. Cargill Incorporated is the assignee of the present application as well.
Other published documents which concern polymers of lactic acid or lactide include: International Publication No. WO 94/06856 to Sinclair et al., published Mar. 31, 1994; International Publication No. WO 92/04413 to Sinclair et al., published Mar. 19, 1992; and International Publication No. WO 90/01521 to Sinclair et al., published Feb. 22, 1990.
It has been reported that lactic acid residue containing polymers, if not treated, are too unstable for processing at high temperatures. See U.S. Pat. No. 5,338,822 to Gruber et al.
There exist three conventional techniques for forming linear polyesters. These techniques include the condensation reaction of a diacid with a diol; the condensation reaction of an hydroxy-acid and/or ester; and the ring opening polymerization of a cyclic ester. Although the polymer resulting from each technique contains the characteristic ester linkage, the structure of the polymer can be different. In the technique involving the condensation reaction of a diacid with a diol, the repeat unit will have a head-tail-tail-head configuration as shown by the general formula: --[(CR.sub.2).sub.x --O--CO--(CR'.sub.2).sub.y --CO--O--]. In contrast, the techniques involving the condensation of an hydroxy acid and/or ester and the ring opening polymerization of a cyclic ester provide a polymer having a simpler repeat formula and a head-tail-head-tail structure as shown by the general formula: --[(CR".sub.2).sub.y --CO--O]--. This difference is similar to the difference between Nylon 6,6 and Nylon 6.
Polyesters produced by condensation reaction of an hydroxy-acid and/or ester and/or by the ring opening polymerization of a cyclic ester may be depolymerized by an "unzipping" or depolymerization reaction. An exemplary depolymerization reaction is demonstrated by the reverse arrow in the equilibrium reaction for polylactide shown below. ##STR1## The possibility exists for copolymers where each of the monomer units can be drawn in a cyclic form. In the case of a polymer formed by polymerizing lactic acid, the depolymerization reaction can lead to the formation of lactide, a cyclic ester (diester).
The formation of lactide by depolymerization during melt processing is undesirable. At a certain level, the formation of lactide can cause a change in the physical properties of the molten polylactide polymer such as reducing the polymer melt viscosity and melt elasticity. In addition, the lactide is volatile and can result in fuming and/or fouling of the processing equipment. Furthermore, depolymerized lactide in the final product is likely to cause decreased shelf life. Lactide is generally more susceptible to hydrolysis, leading to acidity in the final product which can catalyze polymer hydrolysis. In certain applications, high lactide levels may raise concerns about migration into foodstuffs.
The tendency to form lactide is also a potential source of difficulty in preparing a polylactide composition with a low residual lactide level. Residual lactide can be removed at elevated temperatures under vacuum. However, the tendency to form additional lactide will be a competing reaction, giving a composition with a higher residual lactide level. Additional lactide may also form in the equipment and piping following the devolatilization zone. Accordingly, it is important to minimize the time the polymer is exposed to high temperature following the devolatilization zone.
The formation of the cyclic ester (lactide) can be considered the "reverse reaction." It is, in fact, the reverse reaction for the ring-opening polymerization reaction of lactide polymerization, but it can also be considered the reverse reaction in the case of lactic acid polymerization through direct condensation. The propensity to form cyclic ester by depolymerization has been found to be related to catalyst concentration and activity, and to the equilibrium of the reaction as described in U.S. Pat. No. 5,338,822 to Gruber et al. which issued on Aug. 16, 1994 and by Witzke et al., Macromolecules, 30, 7075-7085, 1997.
There exists comparatively little literature discussing the equilibrium relationships for most of the ring systems compared with non-ring systems. Lactide is one of the better known ring systems, and the equilibrium relationship between lactide and polylactide has been described in various references, including U.S. Pat. No. 5,338,822 to Gruber et al. Other ring systems have been tested to determine the general feasibility of polymerization and results reported in references such as M. H. Hartmann, High Molecular Weight Polylactic Acid Polymers, Chapter 15, pp. 367-411, Biopolymers from Renewable Resources, Kaplan, D. (ed.), Springer-Verlag, Berlin, to be published July 1998; R. D. Lundberg and E. F. Cox, Lactones, Chapter 6, pp. 247-302, Kinetics and Mechanisms of Polymerization, vol. 2, 1969; D. B. Johns, R. W. Lenz and A. Luecke, Lactones, Chapter 7, pp. 461-521, Ring-Opening Polymerization, 1984; Y. Chujo and T. Saegusa, Ring-Opening Polymerization, pp. 662-647, Encyclopedia of Polymer Science and Eng., vol. 1, 1988, John Wiley & Sons, New York. A ring system which tends to not polymerize in the presence of catalyst under elevated temperature can be described as a stable ring system. In such a system, the equilibrium condition is far to the left in the equilibrium reaction shown above. Systems which do not polymerize are of little concern because polymer does not form. A system which polymerizes readily corresponds to a system which goes far to the right. These types of systems are generally inherently stable against depolymerization via cyclic ester formation. The systems which polymerize to a certain extent, but not completely, are of the greatest concern. This is because once they have been successfully polymerized there may exist a strong tendency to depolymerize during melt processing, forming the cyclic esters and incurring the problems described above.
Prior attempts at stabilizing polylactide polymers include end capping the polymer, removing or precipitating catalyst from the polymer, controlling the catalyst level, or deactivating the catalyst. The use of certain phosphite compounds as chain extension agents has been noted in the literature for use in polyethylene terephthalate and some other polyesters. See, for example, U.S. Pat. Nos. 4,417,031 and 4,568,720, and Aharoni et al., J. Poly. Sci. A, vol. 24, p. 1281-1296, (1986).